This invention relates to an electron beam condensing electron lens which is primarily used as an electron microscope illumination lens, wherein a single pole gap or a double pole gap can be selected as desired.
The remarkable progress of electron microscopy in recent years permits users to observe not only transmission images but also scanning images with the use of a single electron microscope. In such an electron microscope, it is necessary that the specimen be irradiated by a relatively broad beam, the elements of which are substantially parallel in order to obtain transmission images. On the other hand, a specimen must be irradiated by a very finely focused beam to obtain scanning images. To meet these requirements, it is customary to use, as an illumination lens system, a two-lens system to obtain transmission images, and a three-lens system to obtain scanning images.
FIG. 1 shows a ray diagram of an illumination lens system having a first condenser lens of single-gap construction. FIG. 2 illustrates a ray diagram of an illumination lens system having a first condenser lens of double-gap construction. Each of the illumination lens systems includes a first condenser lens 1, a second condenser lens 2, and an electron gun 3. Object 4 refers to the specimen to be observed. In each of the illumination lens systems shown in FIGS. 1 and 2, an electron beam emitted from the electron gun 3 is condensed by the first and second condenser lenses 1, 2; it then impinges on the specimen. However, the first condenser lens 1 of double-gap construction shown in FIG. 2 comprises two lenses, 1a and 1b.
FIG. 3 illustrates the relation between magnification .vertline.M.vertline. of the illumination lens system and the reciprocals of the focal length (1/f) of the first condenser lenses 1 of single-gap construction and double-gap construction (see FIGS. 1 and 2). Designated as S is a curve for the single-gap lens, and D (D1, D2, D3) is a curve for the double-gap lens. Since the reciprocal of the focal length (1/f) is proportional to the square J.sup.2 of the excitation intensity J for the condenser lenses, the horizontal axis indicates J.sup.2 as well as 1/f. The vertical axis indicates "dc" (beam spot size) as well as magnification, since the spot size/diameter of the electron beam, which impinges on the specimen, is the product of the magnification .vertline.M.vertline. and the size of the crossover of the electron beam source.
As is clear from FIG. 3, the spot size dc of the electron beam obtained by using the double-gap lens is much smaller than that obtained by using the single-gap lens at the same practical excitation intensity.
Therefore, the double-gap lens is preferable for observing scanning images generated by an electron beam having a smaller diameter.
On the other hand, it is known that the single-gap lens is preferable for observing transmission images formed by an electron beam having a larger diameter in order to obtain a parallel beam.
FIG. 4 shows a vertical cross-section of a conventional first condenser lens of double-gap configuration. The first condenser lens 1 as shown comprises a yoke 5, which defines a magnetic circuit, a pole piece 6 disposed in the magnetic circuit and having first and second gaps 6a, 6b, annular spacers 7a, 7b, of nonmagnetic material which define the first and second gaps 6a, 6b, respectively, and an excitation coil 8. The prior double-gap lens is disadvantageous in that conversion from double-gap construction into a single-gap configuration requires disassembly of the lens barrel and replacement of the lens itself, or the changing of some parts of the lens for replacement of the pole piece, a procedure which is quite time-consuming and laborious.